A conventional high-frequency circuit is configured by mounting a plurality of transistors arranged and formed on a semiconductor substrate in parallel to each other and matching circuits arranged on an insulating substrate in a package.
The matching circuits are configured by one input matching circuit and one output matching circuit. The input matching circuit is commonly connected to gate terminals of the plurality of transistors, and the output matching circuit is commonly connected to drain terminals of the plurality of transistors.
When high-frequency signals are input to the conventional high-frequency circuit, some high-frequency signals oscillate at various oscillation frequencies depending on frequency bands held by the signals in the circuit.
For this reason, for example, in order to suppress a signal from oscillating at a GHz-order high oscillation frequency, each of an input matching circuit and an output matching circuit is divided into a plurality of circuits, and the divided matching circuits are connected to each other with resistors (isolation resistors). Oscillation at a high oscillation frequency occurs in a closed loop (electrically closed path for a high-frequency signal) formed by two adjacent transistors, input matching circuits connected to the transistors, and output matching circuits connected to the transistors. This closed loop includes a position closest to a transistor between the input matching circuits and between the output matching circuits. Therefore, the isolation resistor is arranged at only a position closest to the transistor between the matching circuits.
Furthermore, for example, in order to suppress oscillation from occurring at a low MHz-order oscillation frequency, low-frequency oscillation suppressing circuits are connected to gate terminals of transisotors, located on both the sides, of the plurality of transistors which are arranged in parallel to each other. The low-frequency oscillation suppressing circuit is configured by connecting an inductor L, a resistor R, and a capacitor C in series with each other.
However, in the conventional high-frequency circuit, a plurality of isolation resistors are arranged from a transistor located near the center to the low-frequency oscillation suppressing circuit. Therefore, a resistance of a path through which a signal is transmitted between the transistor near the center and the low-frequency oscillation suppressing circuit increases. Therefore, the low-frequency oscillation suppressing circuit is not substantially connected to the transistor located near the center. For this reason, the low-frequency oscillation suppressing circuit does not act on the transistor located near the center. Therefore, there is a problem that it is difficult for the conventional high-frequency circuit to suppress oscillation at a low oscillation frequency.
Furthermore, in order to avoid the problem, when each of the input matching circuit and the output matching circuit is not divided into a plurality of circuits, it is difficult to suppress oscillation at a high oscillation frequency.
More specifically, in a conventional high-frequency circuit, it is difficult to suppress oscillation at a high oscillation frequency and oscillation at a low oscillation frequency.